When designing microwave circuits, transmission lines are commonly used. A transmission line is normally formed on a dielectric carrier material. Due to losses in the dielectric carrier material, it is sometimes not possible to use any transmission lines. When there for example is a filter component in the layout, it may have to be realized in waveguide technology. Waveguides are normally filled with air or other low-loss materials.
Despite quite impressive progress demonstrated in the last few decades in the microwave engineering area, the important role of waveguide components remains undisputed, this is due to their low loss and high power capability performance.
A waveguide E-plane filter component normally comprises two main parts, a first main part comprising a first waveguide section part and a second main part comprising a second waveguide section part. Each waveguide section part comprises three walls; a bottom and corresponding sides.
The first main part and the second main part are arranged to be mounted together such that the first waveguide section part and the second waveguide section part face each other, and together constitute a resulting waveguide section part. This means that each main part comprises a half-height waveguide section part where, when mounted together, the resulting waveguide section part constitutes a full-height waveguide section part.
The electromagnetic field propagates parallel to the intersection. Since the waveguide section part normally have equal sizes, and thus the same height of the corresponding sides, the dominant TE10 mode of the electromagnetic field has its maximum magnitude at said intersection.
Between the main parts, at the intersection, an electrically conducting foil is placed, having a filter part comprising full height or partial-height apertures. The filter part runs between the waveguide section parts.
In order to improve the spectral selectivity and stop-band attenuation, a class of filters for which an amplitude transfer function has attenuation poles at finite frequencies is used. The transmission zeros, attenuation poles, at finite frequencies can be introduced by cross-coupling resonant cavities. Since this solution is not always realizable, the transmission zeroes at the finite frequencies can by introduced using band-stop resonators. Each band-stop resonator allows one to realize one transmission zero either below or above the pass-band of the filter. An E-plane band-stop resonator is usually realized in the form of a T-junction with one port being short-circuited. Such a T-junction is comprised in the main parts with the conductive foil disposed in between the main parts, realizing the coupling between the band-stop cavity and the rest of the E-plane filter.
These T-junctions constitute so-called extracted cavities, allowing realization of said transmission zeroes. These extracted cavities are constituted by relatively small confined openings.
Generally, the benefit of an E-plane filter is that the same main parts can be used for the filters working at different center frequencies and/or covering different bandwidths at different frequency bands. This may be achieved by using the same main parts and change the electrically conducting foil to one having the aperture configuration that provides the desired frequency characteristics.
However, when a waveguide filter design based on E-plane technology is concerned, the relative positions of extracted cavities in the form of said T-junction need to be fixed. The distance between a common port of the waveguide filter and an extracted cavity thus needs to be fixed for a given frequency characteristic of the waveguide filter. This limits the possibility of having the same main parts and replacing the conductive foil disposed between the main parts to realize different filter characteristic.
It has also been proposed to have a foil that comprises at least one foil loop constituted by a foil conductor having a starting point and an end point. The foil conductor is running in a corresponding aperture in the foil. Although advantageous, this arrangement results in an increased length of the foil as well as higher demands on manufacturing processes, especially for small foil loops.
There is thus a desire to obtain a microwave waveguide E-plane filter structure, where the structure may be used for different center frequencies and/or frequency bands by only changing the electrically conducting foil according to the above, but with enhanced properties with respect to prior art.